The Difficulty Class (DC) for all spells is 10 + Caster Level + Caster's MIND bonusFair enough, this is right out of the 3.5 SRD. This is a somewhat OK rule for determining if creatures resist magic, but not so good a rule for if players resist magic. Unless you really want to stat out all your NPC casters or use monster HD for their MIND bonus (which could be perfectly acceptable), then it adds unnecessary complication. Also 10 is a normal difficulty, I think resisting a spell should be somewhat harder than say hearing an approaching guard. You should be breaking a sweat anyway. Also I can see a player's MIND stat influencing their ability to successfully cast a spell and to resist a spell - however once a spell is cast I figure it's a done deal - the magic horse has left the barn - and so the caster's MIND bonus is not likely to influence that target's ability to resist it's effects. Let's give the dimwitted spell casters a break on this one.
So this is what I say about resisting spells:
For spells that can be resisted, the resisting target must match the Difficulty Class (DC) of 15 + Caster Level.I'd almost be tempted to use the spell level instead of the caster level because otherwise you are going to have to figure out the caster's level for scrolls, wands, magic traps and all matter of things. If you just said DC was 15+2*spell level you could have everything laid out right on the table. Of course you would then have capped resistance to low level spells. A 15th level caster can cast a sleep spell just like a 1st level caster can - should the target be resisting that sleep spell against DC 30 or DC 17? I guess it depends on how you think magic should work (and in particular point based magic).